Radiant source tracker independent of non-constant irradiance

ABSTRACT

A tracker of a radiant source is disclosed, comprising a sectored matrix of detectors whose output voltages correspond to their respective levels of irradiance. The voltages are each sampled during a variable length period and the resulting samples are shaped into triangular samples by an integration process. They are supplied to a demodulator which provides an output corresponding to the difference of the time-voltage product of appropriate combinations of the samples. This output after filtering represents the angle of incidence on the detectors with respect to a central axis. The triangular samples are also supplied to a demodulator whose output is an AGC voltage which is a function of the sum of the time-voltage integrals of the triangular samples. This AGC voltage is supplied to a pulse width modulator whose output controls the period of the voltage sample which is taken from each detector.

United States Patent 91 Fletcher et al.

[ 51 Mar. 27, 1973 RADIANT SOURCE TRACKER INDEPENDENT OF NON-CONSTANTIRRADIANCE Primary Examiner-James W. Lawrence Assistant Examiner-D. C.Nelms Attorney-John R. Manning et al.

[76] Inventors: James C. Fletcher, Administrator of the NationalAeronautics and Space [57] ABSTRACT Administration in respect to theinvention f; Fred c n L05 A tracker of a radiant source is disclosed,comprising Angeles, Calif. a sectored matrix of detectors whose outputvoltages correspond to their respective levels of irradiance. [22]F'led: 1971 The voltages are each sampled during a variable [21] APPLNOJ212 900 length period and the resulting samples are shaped intotriangular samples by an integration process. They are supplied to ademodulator which provides an [52] US. Cl ..250/203 R, 250/833 H,250/214, output corresponding to the difference of the time 35 6/152voltage product of appropriate combinations of the [51] lllt. Cl ..G0l1/20 samples This output after filtering represents the Field ofSeaI'Ch-ZSO/ZOZ H; angle of incidence on the detectors with respect to a356/141 172 central axis. The triangular samples are also supplied to ademodulator whose output is an AGC voltage References C'ted which is afunction of the sum of the time-voltage in- UNITED STATES PATENTS tegrals of the triangular samples. This AGC voltage 18 supplied to a pulsewidth modulator whose output 3,479,604 11/1969 Abernathy ..328/147controls the period of the voltage sample which is 3,671,748 6/1972Friedman ..250/203 taken from each detector 3,418,478 12/1968 Falbel..250/203 3,521,071 7/1970 Speller ..250/203 Claims, 7 Drawing Figures22b 22c 22b8 2209 32 35 2243 h 52% -22 g -/1- l2 I4 22 2o 22s 254 LDEEECTOR l NETWORK VA SAMPLE LOW PASS A 5 SAMPLER SHAPER DEMOD' FILTERTRACKING 1: ERROR T J SIGNAL IO l3 IS F 1 1 DETECTOR M LOAD V NETWORK aSAMPLER CI iRL 8 JLJL Z6 F4: 43-. I l m l I JUUL ll cs AT E i I l +2 44I CLOCK T DEMOD. I INPUT "AND" 1 42 GATE T t I I PULSE I i w, WIDTH IMOD 1 I PAT[NlLDH.\R27|9l$ 3,723,745

SHEET 2 BF 4 FIG. 3

OUTPUTS HIGH IRRADlANCE LOW IRRADIANCE VA m A m V VB f f GATE 43 n GATE44 Fl o o SAMPLER H I4 0 Q I l SAMPLER 5 o m o l SHAPERZO o /l o DEMOD30 A O O w FILTER 34 w FRED D. CAMPBELL lNI/IEN'I'UR.

V ATTORNEYS PATENTEU MARE! I973 3. 723, 745

SHEET 3 [IF 4 FIG. 4 a

A 50 A 34 l f I FROM 25 SYNC L Low GATED 52 SUMMING PASS 1 cmcun FILTERTRACKING 4] ERROR SIGNAL FROM 25 INVERTER 4 TO 14 a 15 T AGC VOLTAG E xLOW A SYNC- FRED D CAMPBELL PAS s GATE 0 INVENFOR. F'LTER SUMMINGCIRCUIT %X7 /V/W ATTORNEYS RADIANT SOURCE TRACKER INDEPENDENT FNON-CONSTANT IRRADIANCE ORIGIN OF INVENTION BACKGROUND OF THEINVENTION 1. Field of the Invention The present invention is generallydirected to a radiant source tracker and, more particularly, to atracker whichtracks a source of non-constant radiance.

2. Description of the Prior Art As used herein a tracker is assumed todefine a device which develops an output signal which is a function ofthe angle between the device s axis and the lineof-sight to a source ofradiance, which is in the devices field of view. A common problemassociated with prior art trackers, falling in this generic class, liesin the normalization of the transfer function of the signal for a rangeof irradiance levels into its aperture. Briefly, it is desired that theoutput tracking error signal be a function only of angular trackingerror. However, as is known by those familiar with the art, this is notthe case in prior art trackers. Therein, the change of the outputtracking error signal as a function of change in angular tracking erroris greatly dependent on the radiance level of the source, which is amarked disadvantage, particularly if the level changes are not known.Also most prior art trackers employ mechanical moving parts orelectronic or mechanical scanning techniques which increase the trackerscomplexities, sizes and costs. Thus, a need exists for a tracker whichprovides an output tracking error signal, which is independent of thelevel of the radiance from the source which is tracked, and which is notlimited by other disadvantages of prior art devices.

OBJECTS AND SUMMARY OF THE INVENTION It is a primary object of thepresent invention to pro vide a new improved tracker of a source ofradiance.

Another object of the present invention is to provide a new trackerwhich provides a tracking error signal which is dependent only onangular tracking error.

A further object of the present invention is the provision of a trackerof a source of non-constant radiance.

Yet another object of the present invention is to provide a trackerwhich does not employ mechanically moving parts.

Still another object of the present invention is to provide a trackerwhich does not employ electronic or mechanical scanning of either thesource or the field of view.

A further object of the present invention is to provide a tracker whichcan track sources which have a widely varying angular subtense whenviewed from the tracker.

Yet a further object of the present invention is to provide a trackerwhich can track in two-axes simultaneously.

These and other objects of the present invention are achieved byproviding a tracker with a matrix of detectors. The radiant flux fromthe source to be tracked is imaged onto the detectors. The spatialextent of the radiant flux is such that some energy falls on each of thedetectors when the source is in the trackers linear field of view. Theoutputs of each of the detectors are sampled during equal duration timeperiods. These can be successive in time or simultaneous. The trackerincludes means which derives the difference between appropriatecombinations of samples from the detectors to obtain the tracking errorsignal with respect to one or more central axes. The samples are alsoaveraged to provide an automatic gain control (AGC) signal which is usedto adjust the duration of the sampling period, thereby making thetracking error signal independent of radiance level changes." 7 V V Thenovel features of the invention are set forth with particularity in theappended claims. The invention will best be understood from thefollowing description when read in conjunction with the accompanyingdrawings.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a block diagram of thepresent invention for tracking about a single axis;

FIG. 2 is a diagram illustrating the characteristics of the AGCdemodulator, shown in FIG. 1;

FIG. 3 is a multiline waveform diagram of samples and pulses useful inexplaining the operation of the tracker shown in FIG. 1 for twodifferent radiance levels;

FIG. 4 is a diagram useful in explaining the advantages of shapingsquare samples into triangular pulses;

FIG. 5 is a block diagram of another embodiment of the tracker;

FIG. 6 is a partial diagram of another embodiment; and

FIG. 7 is a matrix detector diagram for two-axes tracking.

DESCRIPTION OF THE PREFERRED EMBODIMENTS Hereafter the invention willfirst be described in conjunction with a single axis electro-opticaltracker for tracking a radiant source. The implementation first to bedescribed employs sequential sampling of the detected signals.Simultaneous sampling employing parallel processing and differentdemodulators will be described thereafter. Two-axes tracking employingfour detectors will also be described.

As will be appreciated the invention is not limited to the specificimplementations described and referenced. This novel tracking conceptmay be employed to pro vide a signal which is a function of angle withrespect to any type of emitter of non-constant radiant energy or anyemitter whose angular subtense, from the tracker, is varying ornon-homogeneous.

As seen from FIG. 1, in one embodiment the tracker includes two opticaldetectors A and B with their light sensitive areas adjacent to oneanother. In practice an image created by the radiant flux from the lightsource, which is being tracked, is imaged on the detectors. Deliberatedefocussing may be employed to enlarge the image and extend the linearfield-of-view. The spatial extent of the image, represented by arrow 10,is such that some energy falls on each of the detectors when the sourceis within the trackers linear field-of-view.

The outputs of detectors A and B are supplied to detector load networks12 and 13, respectively. Each of these networks provides an outputvoltage which is a function of the light detected by the detector towhich it is connected. As shown, each of the two networks comprises acapacitor C and load resistor R The relative values of the components ofeach of the networks may differ as required to accommodate detectors ofdiffering sensitivity. The voltage across resistors R of networks 12 and13 are designated V A and V respectively.

These voltages are successively sampled by samplers 1'4 and"15 which areactivated by pulses from a network 18, whose mode of operation will bedescribed hereafter in detail. Briefly, network 18 controls samplers 14and 15 to sample V and V,, respectively, during two successive equalduration periods. The period is controlled as a function of the netirradiation of detectors A and B. The successive samples of the samplers14 and 15 are shaped by a sample shaper 20.

In FIG. 1, the input to shaper 20 is represented by a succession ofsamples in the form of pulses. The succession or sequence is designatedby numeral 22. Numerals 22a designate the samples from sampler l4 andnumerals 22b designate the samples from sampler 15. The samples areshaped into a function which permits the maximization of the dynamicinput range over which the tracker will function. In one embodiment,shaper 20 is an integrator thereby converting each rectangular sample ofsequence 22 into a corresponding triangular sample. The output samplesequence of shaper 20 is designated by 22S and each sample therein isdesignated by its corresponding input sample followed by the letter S.

The output sample 228 is then amplified by an amplifier 25 whose outputdrives a demodulator 30. Basically, demodulator is a synchronousdetector which produces an output which is proportional to thedifference between the two samples corresponding to a detector pair suchas samples 22as and 22bs. Herein, difference is defined as thedifference of the time-voltage integral of the two samples. The outputof demodulator 30 is designated by the sample sequence 32.

Alternately viewed, the demodulator 30 provides an output in which thepolarity of the samples 22bs are inverted in polarity compared with thesamples 22as. The output of demodulator 30 is amplified and filtered bya lowpass filter 34, which removes the frequencies which are generatedby the preceding sampling operation, leaving only the components whichcarry the tracking error information. The tracking error is a DC signalwhose amplitude depends on the difference between successive pairs ofthe demodulators output samples, which, as is appreciated, are relatedto the irradiance difference on detectors A and B respectively. Thepolarity of the error signal depends on which of the samples is greater.

In FIG. 1, it is assumed that the irradiance of detector A is greaterthan that of B. Therefore, samples 22as are greater than samples 22bsand consequently the output error signal has a plus polarity. The outputerror signal with respect to a reference potential, such as ground (0volt), is designated by numeral 35.

As seen from FIG. 1, the output of amplifier 25 is also supplied to anautomatic gain control (AGC) demodulator 40. This demodulator produces avoltage which is proportional to the average of the time-voltageintegral products, represented by the areas of the samples in sequence228. This output voltage will be referred to as the AGC voltage. It issupplied to a pulse width modulator 42 which forms part of the network18. Briefly, modulator 42 controls the sampling period of samplers 14and 15. Demodulator 40 is synchronously clocked from the pulse widthmodulator. Demodulation thus occurs only during the detector signalsampling periods.

Network 18 includes a pair of AND gates 43 and 44 and a divide-by-twocircuit 45. The latter, as well as modulator 42 are provided with clockpulses 46 from an appropriate clock (not shown). The divider 45 operatesto divide the clock frequency by two. This new frequency is provided togate 43. Its inverse is pro vided to gate 44. The effect is to enableeach of the gates during successive clock periods. Stated otherwise,successive gating pulses are supplied alternately to the two AND gates.Each clock pulse also activates modulator 42 to supply an enabling pulseto the two gates. The time duration or width of the enabling pulse is afunction of the AGC voltage.

It is thus seen that the samplers 14 and 15 are activated successively,rather than simultaneously, during successive clock pulse periods.However, the sampling period is controlled by the width of the enablingpulse from modulator 42 which is controlled by the AGC voltage. In oneparticular embodiment the characteristic of the modulator is chosen asillustrated in FIG. 2. It is thus seen that the modulator provides aconstant width pulse, which in this embodiment equal one-half the clockpulse period, until the AGC voltage reaches a threshold. As the AGCvoltage starts to exceed the threshold, the pulse width starts todecrease in accordance with a predetermined function, such asexponentially, as shown in fig. 2. As the irradiance level increases theAGC voltage increases and when it exceeds its threshold it causes thesampling period to decrease. Conversely, low irradiance levels cause theAGC voltage to drop below its threshold and thereby increase thesampling period to its maximum value. A constant AGC voltage impliesthat the net area of the samples is constant thereby eliminating theeffects of varying input irradiance levels.

The operation of the novel'tracker of the present invention may best besummarized in conjunction with FIG. 3 wherein the outputs of the variouscircuits herebefore described are shown for high and low irradiancelevels. As is appreciated from the foregoing description at a highirradiance level the sampling period as represented by the outputs ofgates 43 and 44 is shortened, as compared with the sampling periodduring a low irradiance level. Thus the differential area of any pair ofsamples is a function only of the relative proportioning of theirradiance between the two detectors and not a function of the totalmagnitude or level of the irradiance.

As previously stated, the rectangular samples from samplers 14 and 15are shaped by shaper 20 into triangular samples, which are thenamplified to drive demodulator 40 which produces the AGC voltage as afunction of the time integral of all the samples. Such sample shapingpermits gain control over a wide dynamic range of inputs. It avoidssaturation of the amplifier 25 and the demodulators. Saturation of thesecircuits would restrict the trackers dynamic range if rectangularsamples were processed. This aspect is best highlighted in FIG. 4wherein sample modulation for successive doubling of input irradiance isdiagrammed in terms of rectangular samples and their correspondingtriangular samples. All successive samples are of equal area.

As is known, the area of a triangle varies as the square of its basedimension while that of a rectangle varies as a linear function of itsbase dimension. Thus it is seen that the use of the triangular samplesincreases the dynamic range of the AGC by delaying saturation of theamplifier and demodulators. This is true since the peaks of thetriangular samples are lower than those of their correspondingrectangular samples.

Although triangular samples have been described it is believed thatsamples with exponentially increasing voltages would further increasethe effective dynamic range of the tracker. This could be accomplishedby double integration of the original rectangular sample pulse. However,this is at the cost of increased complexity of the shaper 20.

Although herebefore the invention has been described in connection witha sequential sampling scheme it is not limited thereto. Parallelsimultaneous sampling can also be employed by providing a separateamplifier for each additional channel and using a different type ofdemodulator for demodulator 30. A simple block diagram for parallelsimultaneous sampling is shown in FIG. 5 wherein elements like thoseshown in FIG. 1 are designated by like numerals. As seen in FIG. 5, theoutput samples of sampler are shaped by a shaper 200 which is identicalwith shaper herebefore described. The output of shaper 20a whichconsists of triangular pulses are amplified by amplifier a, whose outputtogether with that of amplifier 25 are supplied to demodulator 40. Thelatter generates the AGC voltage as a function of the sum of the timeinterval of the triangular pulses from both amplifiers. The outputs ofthe amplifiers 25 and 25a are supplied to demodulators a and 30brespectively. These provide the time voltage integrals or products ofthe triangular samples of the two amplifiers. The outputs of the twodemodulators are subtracted by subtractor 34a whose output representsthe tracking error signal. Thus, herein demodulators 30a and 30b andsubtractor 34a perform the functions performed by demodulator 30 andfilter 34 shown in FIG. 1.

As shown in FIG. 6, the demodulators 30a and 30b and subtractor 34a maybe replaced by a synchronously gated summing circuit 50, an inverter 52and a lowpass filter, such as filter 34. Also demodulator 40 may bereplaced by a synchronously gated summing circuit 55 and a lowpassfilter 56. Both circuits 50 and 55 are gated by the variable-time pulsesfrom the pulse width demodulator 42. The aforedescribed embodiments areapplicable for single-axis tracking. If desired the teachings may beemployed for two-axes tracking.

In such an application four detectors may be employed as shown in FIG.7. Therein, detectors A and A' are arranged to provide tracking aboutthe horizontal axis H and detectors B and B are used to provide trackingabout the vertical axis V. The irradiance or iilumination is assumed tobe directed to each pair of the detectors by means of a beam splitter(not shown). Each pair of detectors is associated with separatecircuitry as herebefore described.

Another application involves the employment of the principles of theinvention in conjunction with a 4- quadrant detector in a two-axistracker.

Although particular embodiments of the invention have been described andillustrated herein, it is recognized that modifications and variationsmay readily occur to those skilled in the art and consequently it isintended that the claims be interpreted to cover such modifications andequivalents.

What is claimed is:

1. A source-of-energy tracker comprising:

first and second energy detectors for converting energy received fromsaid source about a first axis to first and second outputs respectivelyas a function of the received energy;

sampling means for sampling said first and second outputs to providesamples of said outputs;

output means responsive to said samples for providing an output signalwhich is a function of the relationship between pairs of samples, eachpair including one sample of said first output and a second sample ofsaid second output; and

control means responsive to said samples for controlling the duration ofeach sampling of said first and second outputs as a function of saidsamples.

2. The arrangement as recited in claim 1 wherein said output meansinclude means for providing said output signal as a function of thedifference of the timevoltage characteristics of each pair of samples,and said control means include demodulator means for providing a controlsignal whose amplitude is a function of the sum of time-voltagecharacteristics of samples of said first and second outputs.

3. The arrangement as recited in claim 2 wherein said sampling meansincludes means for providing rectangularly shaped samples of said firstand second outputs of durations controlled by said control means andshaping means responsive for converting said rectangularly shapedsamples to samples of lower amplitudes but equal time-voltageproperties.

4. The arrangement as recited in claim 3 wherein said shaping meansconvert said rectangularly shaped samples into triangularly shapedsamples of corresponding equal areas.

5. The arrangement as recited in claim 1 wherein said control meansinclude means for controlling said sampling means to successively samplesaid first and second outputs during two successive equal durationperiods, the durations of said periods being a function of said samplesto which said control means is responsave.

6. The arrangement as recited in claim 5 wherein said sampling meansinclude means for combining said samples of said first and secondoutputs into a sequence of samples, with alternate samples in saidsequence being from the same output and said output means include meansfor providing said output signal as a function of the difference betweeneach pair of successive samples in said sequence, and said control meansbeing a function of the time-voltage characteristics thereof, and saidcontrol means include demodulator means for integrating said samples insaid sequence to provide said control signal, with an amplitude which isa function of the integrated samples.

8. The arrangement as recited in claim 7 wherein said sampling meansinclude shaping means for shaping each of said samples in said sequence.

9. The arrangement as recited in claim 8 wherein each output sample isof substantially constant amplitude during the entire duration thereofand said shaping means include means for integrating each constantamplitude sample.

10. The arrangement as recited in claim 9 wherein said demodulator meansintegrate said samples to provide an output voltage which is a functionof the timevoltage integral of said samples, said pulse width meansbeing responsive to said output voltage for controlling said samplingduration.

11. A tracker for tracking a source of energy and for providing atracking error signal which is a function of the angle of incidence ofthe energy from said source, comprising;

first and second energy detectors for converting energy received fromsaid source to first and second signals whose amplitudes are related tothe energy detected by said first and second detectors,

respectively;

sampling means for providing a sequence of samples of said first andsecond signals, alternate samples in said sequence being of the samplesignal;

output means responsive to said succession of samples for providing anerror signal whose amplitude is a function of the time-amplitudedifference of a pair of adjacent samples in said sequence; and

control means responsive to said succession of samples for controllingthe durations of the samples provided by said sampling means.

12. The arrangement as recited in claim 11 wherein said control meansinclude a demodulator means for 13. The arrangement as recited in claim12 further including means for shaping the samples in said sequence,into corresponding samples of equal areas and lower amplitudes.

14. The arrangement as recited in claim 12 wherein said sampling meanscomprises first and second samplers for sampling said first and secondsignals respecively during periods controlled by said control means,

means for combining the samples from said first and second samplers,said control means including means for controlling the sampling periodsof said samplers.

15. The arrangement as recited in claim 14 further including means forshaping the samples in said sequence, into corresponding samples withmore constant time bases.

1. A source-of-energy tracker comprising: first and second energydetectors for converting energy received from said source about a firstaxis to first and second outputs respectively as a function of thereceived energy; sampling means for sampling said first and secondoutputs to provide samples of said outputs; output means responsive tosaid samples for providing an output signal which is a function of therelationship between pairs of samples, each pair including one sample ofsaid first output and a second sample of said second output; and controlmeans responsive to said samples for controlling the duration of eachsampling of said first and second outputs as a function of said samples.2. The arrangement as recited in claim 1 wherein said output meansinclude means for providing said output signal as a function of thedifference of the time-voltage characteristics of each pair of samples,and said control means include demodulator means for providing a controlsignal whose amplitude is a function of the sum of time-voltagecharacteristics of samples of said first and second outputs.
 3. Thearrangement as recited in claim 2 wherein said sampling means includesmeans for providing rectangularly shaped samples of said first andsecond outputs of durations controlled by said control means and shapingmeans responsive for converting said rectangularly shaped samples tosamples of lower amplitudes but equal time-voltage properties.
 4. Thearrangement as recited in claim 3 wherein said shaping means convertsaid rectangularly shaped samples into triangularly shaped samples ofcorresponding equal areas.
 5. The arrangement as recited in claim 1wherein said control means include means for controlling said samplingmeans to successively sample said first and second outputs during twosuccessive equal duration periods, the durations of said periods being afunction of said samples to which said control means is responsive. 6.The arrangement as recited in claim 5 wherein said sampling meansinclude means for combining said samples of said first and secondoutputs into a sequence of samples, with alternate samples in saidsequence being from the same output and said output means include meansfor providing said output signal as a function of the difference betweeneach pair of successive samples in said sequence, and said control meansbeing responsive to said sequence, and said control means beingresponsive to said sequence of samples for providing a control signalwhose amplitude is a function of said samples in said sequence, andpulse width means responsive to said control signal for controlling thesampling duration as a function thereof.
 7. The arrangement as recitedin claim 6 wherein each output is a direct-current (DC) voltage and saidoutput means include means for providing a difference between each pairof two successive voltage samples as a function of the time-voltagecharacteristics thereof, and said control means include demodulatormeans for integrating said samples in said sequence to provide saidcontrol signal, with an amplitude which is a function of the integratedsamples.
 8. The arrangement as recited in claim 7 wherein said samplingmeans include shaping means for shaping each of said samples in saidsequence.
 9. The arrangement as recited in claim 8 wherein each outputsample is of substantially constant amplitude during the entire durationthereof and said shaping means include means for integrating eachconstant amplitude sample.
 10. The arrangement as recited in claim 9wherein said demodulator means integrate said samples to provide anoutput voltage which is a function of the time-voltage integral of saidsamples, said pulse width means being responsive to said output voltagefor controlling said sampling duration.
 11. A tracker for tracking asource of energy and for providing a tracking error signal which is afunction of the angle of incidence of the energy from said source,comprising; first and second energy detectors for converting energyreceived from said source to first and second signals whose amplitudesare related to the energy detected by said first and second detectors,respectively; sampling means for providing a sequence of samples of saidfirst and second signals, alternate samples in said sequence being ofthe sample signal; output means responsive to said succession of samplesfor providing an error signal whose amplitude is a function of thetime-amplitude difference of a pair of adjacent samples in saidsequence; and control means responsive to said succession of samples forcontrolling the durations of the samples provided by said samplingmeans.
 12. The arrangement as recited in claim 11 wherein said controlmeans include a demodulator means for providing a control signal whoseamplitude is a function of the time integral of the samples in saidsequence, and means responsive to a succession of clock pulses and tosaid control signal for controlling said sampling means to sample saidfirst and second signals during appropriate pulse periods respectively,the sampling period being a function of the amplitude of said controlsignal.
 13. The arrangement as recited in claim 12 further includingmeans for shaping the samples in said sequence, into correspondingsamples of equal areas and lower amplitudes.
 14. The arrangement asrecited in claim 12 wherein said sampling means comprises first andsecond samplers for sampling said first and second signals respectivelyduring periods controlled by said control means, means for combining thesamples from said first and second samplers, said control meansincluding means for controlling the sampling periods of said samplers.15. The arrangement as recited in claim 14 further including means forshaping the samples in said sequence, into corresponding samples withmore constant time bases.